Inspection apparatus, inspection method, lithographic apparatus, patterning device and manufacturing method

ABSTRACT

Disclosed is a method of monitoring a focus parameter during a lithographic process. The method comprises acquiring first and second measurements of, respectively first and second targets, wherein the first and second targets have been exposed with a relative best focus offset. The method then comprises determining the focus parameter from first and second measurements. Also disclosed are corresponding measurement and lithographic apparatuses, a computer program and a method of manufacturing devices.

BACKGROUND

Field of the Invention

The present invention relates to inspection apparatus and methodsusable, for example, to perform metrology in the manufacture of devicesby lithographic techniques. The invention further relates to suchmethods for monitoring a focus and/or dose parameter in a lithographicprocess.

Background Art

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.,including part of, one, or several dies) on a substrate (e.g., a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned.

In lithographic processes, it is desirable frequently to makemeasurements of the structures created, e.g., for process control andverification. Various tools for making such measurements are known,including scanning electron microscopes, which are often used to measurecritical dimension (CD), and specialized tools to measure overlay, theaccuracy of alignment of two layers in a device. Recently, various formsof scatterometers have been developed for use in the lithographic field.These devices direct a beam of radiation onto a target and measure oneor more properties of the scattered radiation—e.g., intensity at asingle angle of reflection as a function of wavelength; intensity at oneor more wavelengths as a function of reflected angle; or polarization asa function of reflected angle—to obtain a diffraction “spectrum” fromwhich a property of interest of the target can be determined.

Examples of known scatterometers include angle-resolved scatterometersof the type described in US2006033921A1 and US2010201963A1. The targetsused by such scatterometers are relatively large, e.g., 40 μm by 40 μm,gratings and the measurement beam generates a spot that is smaller thanthe grating (i.e., the grating is underfilled). Examples of dark fieldimaging metrology can be found in international patent applicationsUS20100328655A1 and US2011069292A1 which documents are herebyincorporated by reference in their entirety. Further developments of thetechnique have been described in published patent publicationsUS20110027704A, US20110043791A, US2011102753A1, US20120044470A,US20120123581A, US20130258310A, US20130271740A and WO2013178422A1. Thesetargets can be smaller than the illumination spot and may be surroundedby product structures on a wafer. Multiple gratings can be measured inone image, using a composite grating target. The contents of all theseapplications are also incorporated herein by reference.

One important parameter of a lithographic process which requiresmonitoring is focus. There is a desire to integrate an ever-increasingnumber of electronic components in an IC. To realize this, it isnecessary to decrease the size of the components and therefore toincrease the resolution of the projection system, so that increasinglysmaller details, or line widths, can be projected on a target portion ofthe substrate. As the critical dimension (CD) in lithography shrinks,consistency of focus, both across a substrate and between substrates,becomes increasingly important. CD is the dimension of a feature orfeatures (such as the gate width of a transistor) for which variationswill cause undesirable variation in physical properties of the feature.Traditionally, optimal settings were determined by “send-ahead wafers”i.e. substrates that are exposed, developed and measured in advance of aproduction run. In the send-ahead wafers, test structures were exposedin a so-called focus-energy matrix (FEM) and the best focus and energysettings were determined from examination of those test structures.

Current test structure designs and focus measuring methods have a numberof drawbacks. Many test structures require subresolution features orgrating structures with large pitches. Such structures may contravenedesign rules of the users of lithographic apparatuses. Focus measuringtechniques may comprise measuring asymmetry in opposite higher (e.g.,first) order radiation scattered by special, focus dependent, targetstructures and determining focus from this asymmetry. For EUVlithography, resist thickness, and therefore the thickness of targetstructures, is smaller (for example, half as thick). Therefore focussensitivity and signal strength may be insufficient to use suchasymmetry methods in EUV lithography. In addition, asymmetry basedtechniques may require careful selection of target geometries to ensurea desired relationship (e.g., linear) between asymmetry and focus. Thisselection process can be complex and require significant effort to finda suitable target geometry. It may even be the case that no suitabletarget geometry exists.

SUMMARY OF THE INVENTION

The present invention aims to address one or more of the aboveidentified drawbacks.

The invention in a first aspect provides method of monitoring a focusparameter during a lithographic process, said method comprising:

acquiring a first measurement value, said first measurement value havingbeen obtained from inspection of a first target;

acquiring a second measurement value, said second measurement valuehaving been obtained from inspection of a second target,

wherein said first target and second target have been exposed with arelative best focus offset;

determining the focus parameter from said first measurement value andsaid second measurement value.

The invention yet further provides a method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including:

-   -   using the method of the first aspect to monitor said focus        parameter, and controlling the lithographic process for later        substrates in accordance with the determined focus parameter.

The invention yet further provides a computer program product comprisingmachine-readable instructions for causing a processor to perform themethod of the first aspect.

The invention yet further provides a patterning device configured topattern a beam of radiation in a lithographic process according to adesired pattern, said patterning device comprising first features forforming a first target on substrate during the lithographic process, andsecond features for forming a second target on substrate during thelithographic process; wherein said second features are taller than saidfirst features, in a direction transverse to the plane of the target andsuch that said first target and said second target have a relative bestfocus offset.

The invention yet further provides a method of monitoring a doseparameter during a lithographic process, said method comprising:acquiring a first measurement value, said first measurement value havingbeen obtained from inspection of a first target; acquiring a secondmeasurement value, said second measurement value having been obtainedfrom inspection of a second target, determining the dose parameter fromsaid first measurement value and said second measurement value; whereinsaid first and second targets comprise corresponding line and spacetargets having the same pitch and inverse duty cycles.

The invention yet further provides a method of manufacturing deviceswherein a device pattern is applied to a series of substrates using alithographic process, the method including: using the method of theaspect immediately above to monitor said dose parameter, and controllingthe lithographic process for later substrates in accordance with thedetermined dose parameter.

The invention yet further provides a patterning device configured topattern a beam of radiation in a lithographic process according to adesired pattern, said patterning device comprising first features forforming a first target on a substrate during the lithographic process,and second features for forming a second target on the substrate duringthe lithographic process; wherein said first features are configured toform said first target with line features having a substantially focusindependent side wall angle and said second features are configured toform said second target with line features having a focus dependent sidewall angle.

The invention yet further provides a patterning device configured topattern a beam of radiation in a lithographic process according to adesired pattern, said patterning device comprising first features forforming a first target on a substrate during the lithographic process,and second features for forming a second target on a substrate duringthe lithographic process; wherein design rules constrain target featuresto a set critical dimension and on a grid of set pitch, wherein saidfirst target and said second target are each formed from rows of saidgrids such that each of said first features and said second features areformed from one, or plural adjacent, corresponding target featuresformed on said grids.

Further features and advantages of the invention, as well as thestructure and operation of various embodiments of the invention, aredescribed in detail below with reference to the accompanying drawings.It is noted that the invention is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus;

FIG. 2 depicts a lithographic cell or cluster in which an inspectionapparatus according to the present invention may be used;

FIG. 3 illustrates the principles of operation of a spectroscopicscatterometer as a first example of an inspection apparatus;

FIG. 4 illustrates in schematic form an angle-resolved scatterometer asanother example of an inspection apparatus;

FIGS. 5(a) to 5(b) illustrate schematically an inspection apparatusadapted to perform angle-resolved scatterometry and dark-field imaginginspection methods;

FIG. 6 illustrates target forming elements on a reticle suitable forforming a grating on a substrate having focus dependent asymmetry;

FIGS. 7(a) to 7(b) shows (a) a plot of a measured value for a targetparameter (y-axis) against focus for two targets having a relative bestfocus offset; and (b) a plot of the difference between measured valuesfor a target parameter from a first target and a second target (y-axis)against focus (x-axis);

FIGS. 8(a) to 8(d) show schematically, in cross section, possible targetforming designs on a reticle;

FIGS. 9(a) to 9(b) shows schematically, in cross section, (a) a reticleblank according to an embodiment and (b) a further possible targetforming design on a reticle;

FIG. 10 show schematically, in plan view, yet further possible targetforming designs on a reticle;

FIG. 11 is a flowchart of a method of monitoring focus according to anembodiment of the invention;

FIG. 12 show schematically, in plan view, possible target formingdesigns on a reticle for performing dose measurements

FIG. 13 shows a plot of measured intensity (y-axis) against CD (x-axis)for targets such as those illustrated in FIG. 12; and

FIGS. 14(a) to 14(f) each show an example of a possible target featureforming design when constrained by an exemplary grid-based design rule.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Before describing embodiments of the invention in detail, it isinstructive to present an example environment in which embodiments ofthe present invention may be implemented.

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g., UV radiation or DUV radiation), a patterningdevice support or support structure (e.g., a mask table) MT constructedto support a patterning device (e.g., a mask) MA and connected to afirst positioner PM configured to accurately position the patterningdevice in accordance with certain parameters; two substrate tables(e.g., a wafer table) WTa and WTb each constructed to hold a substrate(e.g., a resist coated wafer) W and each connected to a secondpositioner PW configured to accurately position the substrate inaccordance with certain parameters; and a projection system (e.g., arefractive projection lens system) PS configured to project a patternimparted to the radiation beam B by patterning device MA onto a targetportion C (e.g., including one or more dies) of the substrate W. Areference frame RF connects the various components, and serves as areference for setting and measuring positions of the patterning deviceand substrate and of features on them.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device, the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can take many forms, The patterning devicesupport may ensure that the patterning device is at a desired position,for example with respect to the projection system.

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

As here depicted, the apparatus is of a transmissive type (e.g.,employing a transmissive patterning device). Alternatively, theapparatus may be of a reflective type (e.g., employing a programmablemirror array of a type as referred to above, or employing a reflectivemask). Examples of patterning devices include masks, programmable mirrorarrays, and programmable LCD panels. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.” The term “patterning device” can also beinterpreted as referring to a device storing in digital form patterninformation for use in controlling such a programmable patterningdevice.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g., water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems.

In operation, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may for example include an adjuster AD for adjustingthe angular intensity distribution of the radiation beam, an integratorIN and a condenser CO. The illuminator may be used to condition theradiation beam, to have a desired uniformity and intensity distributionin its cross section.

The radiation beam B is incident on the patterning device MA, which isheld on the patterning device support MT, and is patterned by thepatterning device. Having traversed the patterning device (e.g., mask)MA, the radiation beam B passes through the projection system PS, whichfocuses the beam onto a target portion C of the substrate W. With theaid of the second positioner PW and position sensor IF (e.g., aninterferometric device, linear encoder, 2-D encoder or capacitivesensor), the substrate table WTa or WTb can be moved accurately, e.g.,so as to position different target portions C in the path of theradiation beam B. Similarly, the first positioner PM and anotherposition sensor (which is not explicitly depicted in FIG. 1) can be usedto accurately position the patterning device (e.g., reticle/mask) MAwith respect to the path of the radiation beam B, e.g., after mechanicalretrieval from a mask library, or during a scan.

Patterning device (e.g., reticle/mask) MA and substrate W may be alignedusing mask alignment marks M1, M2 and substrate alignment marks P1, P2.Although the substrate alignment marks as illustrated occupy dedicatedtarget portions, they may be located in spaces between target portions(these are known as scribe-lane alignment marks). Similarly, insituations in which more than one die is provided on the patterningdevice (e.g., mask) MA, the mask alignment marks may be located betweenthe dies. Small alignment mark may also be included within dies, inamongst the device features, in which case it is desirable that themarkers be as small as possible and not require any different imaging orprocess conditions than adjacent features. The alignment system, whichdetects the alignment markers is described further below.

The depicted apparatus could be used in a variety of modes. In a scanmode, the patterning device support (e.g., mask table) MT and thesubstrate table WT are scanned synchronously while a pattern imparted tothe radiation beam is projected onto a target portion C (i.e., a singledynamic exposure). The speed and direction of the substrate table WTrelative to the patterning device support (e.g., mask table) MT may bedetermined by the (de-)magnification and image reversal characteristicsof the projection system PS. In scan mode, the maximum size of theexposure field limits the width (in the non-scanning direction) of thetarget portion in a single dynamic exposure, whereas the length of thescanning motion determines the height (in the scanning direction) of thetarget portion. Other types of lithographic apparatus and modes ofoperation are possible, as is well-known in the art. For example, a stepmode is known. In so-called “maskless” lithography, a programmablepatterning device is held stationary but with a changing pattern, andthe substrate table WT is moved or scanned.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

Lithographic apparatus LA is of a so-called dual stage type which hastwo substrate tables WTa, WTb and two stations—an exposure station EXPand a measurement station MEA—between which the substrate tables can beexchanged. While one substrate on one substrate table is being exposedat the exposure station, another substrate can be loaded onto the othersubstrate table at the measurement station and various preparatory stepscarried out. This enables a substantial increase in the throughput ofthe apparatus. The preparatory steps may include mapping the surfaceheight contours of the substrate using a level sensor LS and measuringthe position of alignment markers on the substrate using an alignmentsensor AS. If the position sensor IF is not capable of measuring theposition of the substrate table while it is at the measurement stationas well as at the exposure station, a second position sensor may beprovided to enable the positions of the substrate table to be tracked atboth stations, relative to reference frame RF. Other arrangements areknown and usable instead of the dual-stage arrangement shown. Forexample, other lithographic apparatuses are known in which a substratetable and a measurement table are provided. These are docked togetherwhen performing preparatory measurements, and then undocked while thesubstrate table undergoes exposure.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. Accordingly a manufacturing facility in which lithocell LC islocated also includes metrology system MET which receives some or all ofthe substrates W that have been processed in the lithocell. Metrologyresults are provided directly or indirectly to the supervisory controlsystem SCS. If errors are detected, adjustments may be made to exposuresof subsequent substrates, especially if the inspection can be done soonand fast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped and reworkedto improve yield, or discarded, thereby avoiding performing furtherprocessing on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

Within metrology system MET, an inspection apparatus is used todetermine the properties of the substrates, and in particular, how theproperties of different substrates or different layers of the samesubstrate vary from layer to layer. The inspection apparatus may beintegrated into the lithographic apparatus LA or the lithocell LC or maybe a stand-alone device. To enable most rapid measurements, it isdesirable that the inspection apparatus measure properties in theexposed resist layer immediately after the exposure. However, the latentimage in the resist has a very low contrast—there is only a very smalldifference in refractive index between the parts of the resist whichhave been exposed to radiation and those which have not—and not allinspection apparatus have sufficient sensitivity to make usefulmeasurements of the latent image. Therefore measurements may be takenafter the post-exposure bake step (PEB) which is customarily the firststep carried out on exposed substrates and increases the contrastbetween exposed and unexposed parts of the resist. At this stage, theimage in the resist may be referred to as semi-latent. It is alsopossible to make measurements of the developed resist image—at whichpoint either the exposed or unexposed parts of the resist have beenremoved—or after a pattern transfer step such as etching. The latterpossibility limits the possibilities for rework of faulty substrates butmay still provide useful information.

FIG. 3 depicts a known spectroscopic scatterometer which may be used asan inspection apparatus in a metrology system of the type describedabove. It comprises a broadband (white light) radiation projector 2which projects radiation onto a substrate W. The reflected radiation ispassed to a spectrometer 4, which measures a spectrum 6 (intensity as afunction of wavelength) of the specular reflected radiation. From thisdata, the structure or profile 8 giving rise to the detected spectrummay be reconstructed by calculation within processing unit PU. Thereconstruction can be performed for example by Rigorous Coupled WaveAnalysis and non-linear regression, or comparison with a library ofpre-measured spectra or pre-computed simulated spectra. In general, forthe reconstruction the general form of the structure is known and someparameters are assumed from knowledge of the process by which thestructure was made, leaving only a few parameters of the structure to bedetermined from the scatterometry data. Such a scatterometer may beconfigured as a normal-incidence scatterometer or an oblique-incidencescatterometer.

FIG. 4 shows the basic elements of a known angle-resolved scatterometerthat may be used instead of or in addition to a spectroscopicscatterometer. In this type of inspection apparatus, radiation emittedby a radiation source 11 is conditioned by an illumination system 12.For example, illumination system 12 may include a collimating using lenssystem 12 a, a color filter 12 b, a polarizer 12 c and an aperturedevice 13. The conditioned radiation follows an illumination path IP, inwhich it is reflected by partially reflecting surface 15 and focusedinto a spot S on substrate W via a microscope objective lens 16. Ametrology target T may be formed on substrate W. Lens 16, has a highnumerical aperture (NA), preferably at least 0.9 and more preferably atleast 0.95. Immersion fluid can be used to obtain with numericalapertures over 1 if desired.

As in the lithographic apparatus LA, one or more substrate tables may beprovided to hold the substrate W during measurement operations. Thesubstrate tables may be similar or identical in form to the substratetables WTa, WTb of FIG. 1. (In an example where the inspection apparatusis integrated with the lithographic apparatus, they may even be the samesubstrate tables.) Coarse and fine positioners may be configured toaccurately position the substrate in relation to a measurement opticalsystem. Various sensors and actuators are provided for example toacquire the position of a target of interest, and to bring it intoposition under the objective lens 16. Typically many measurements willbe made on targets at different locations across substrate W. Thesubstrate support can be moved in X and Y directions to acquiredifferent targets, and in the Z direction to obtain a desired focusingof the optical system on the target. It is convenient to think anddescribe operations as if the objective lens and optical system beingbrought to different locations on the substrate, when in practice theoptical system remains substantially stationary and only the substratemoves. Provided the relative position of the substrate and the opticalsystem is correct, it does not matter in principle whether one or bothof those is moving in the real world.

When the radiation beam is incident on the beam splitter 16 part of itis transmitted through the beam splitter and follows a reference path RPtowards a reference mirror 14.

Radiation reflected by the substrate, including radiation diffracted byany metrology target T, is collected by lens 16 and follows a collectionpath CP in which it passes through partially reflecting surface 15 intoa detector 19. The detector may be located in the back-projected pupilplane P, which is at the focal length F of the lens 16. In practice, thepupil plane itself may be inaccessible, and may instead be re-imagedwith auxiliary optics (not shown) onto the detector located in aso-called conjugate pupil plane P′. The detector is preferably atwo-dimensional detector so that a two-dimensional angular scatterspectrum or diffraction spectrum of a substrate target 30 can bemeasured. In the pupil plane or conjugate pupil plane, the radialposition of radiation defines the angle of incidence/departure of theradiation in the plane of focused spot S, and the angular positionaround an optical axis O defines azimuth angle of the radiation. Thedetector 19 may be, for example, an array of CCD or CMOS sensors, andmay use an integration time of, for example, 40 milliseconds per frame.

Radiation in reference path RP is projected onto a different part of thesame detector 19 or alternatively on to a different detector (notshown). A reference beam is often used for example to measure theintensity of the incident radiation, to allow normalization of theintensity values measured in the scatter spectrum.

The various components of illumination system 12 can be adjustable toimplement different metrology ‘recipes’ within the same apparatus. Colorfilter 12 b may be implemented for example by a set of interferencefilters to select different wavelengths of interest in the range of,say, 405-790 nm or even lower, such as 200-300 nm. An interferencefilter may be tunable rather than comprising a set of different filters.A grating could be used instead of interference filters. Polarizer 12 cmay be rotatable or swappable so as to implement different polarizationstates in the radiation spot S. Aperture device 13 can be adjusted toimplement different illumination profiles. Aperture device 13 is locatedin a plane P″ conjugate with pupil plane P of objective lens 16 and theplane of the detector 19. In this way, an illumination profile definedby the aperture device defines the angular distribution of lightincident on substrate radiation passing through different locations onaperture device 13.

The detector 19 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

Where a metrology target T is provided on substrate W, this may be a 1-Dgrating, which is printed such that after development, the bars areformed of solid resist lines. The target may be a 2-D grating, which isprinted such that after development, the grating is formed of solidresist pillars or vias in the resist. The bars, pillars or vias mayalternatively be etched into the substrate. This pattern is sensitive tochromatic aberrations in the lithographic projection apparatus,particularly the projection system PS. Illumination symmetry and thepresence of such aberrations will manifest themselves in a variation inthe printed grating. Accordingly, the scatterometry data of the printedgratings is used to reconstruct the gratings. The parameters of the 1-Dgrating, such as line widths and shapes, or parameters of the 2-Dgrating, such as pillar or via widths or lengths or shapes, may be inputto the reconstruction process, performed by processing unit PU, fromknowledge of the printing step and/or other scatterometry processes.

In addition to measurement of parameters by reconstruction, angleresolved scatterometry is useful in the measurement of asymmetry offeatures in product and/or resist patterns. A particular application ofasymmetry measurement is for the measurement of a focus parameter (forexample, the focus setting during exposure of the target) from targetswhich print with a focus dependent asymmetry. The concepts of asymmetrymeasurement using the instrument of FIG. 3 or 4 are described forexample in published patent application US2006066855A1 cited above.Simply stated, while the positions of the diffraction orders in thediffraction spectrum of the target are determined only by theperiodicity of the target, asymmetry of intensity levels in thediffraction spectrum is indicative of asymmetry in the individualfeatures which make up the target. In the instrument of FIG. 4, wheredetector 19 may be an image sensor, such asymmetry in the diffractionorders appears directly as asymmetry in the pupil image recorded bydetector 19. This asymmetry can be measured by digital image processingin unit PU, and from this, focus can be determined.

FIG. 5(a) shows in more detail an inspection apparatus implementingangle-resolved scatterometry by the same principles as the apparatus ofFIG. 4, with additional adaptations for performing so-called dark fieldimaging. The apparatus may be a stand-alone device or incorporated ineither the lithographic apparatus LA, e.g., at the measurement station,or the lithographic cell LC. An optical axis, which has several branchesthroughout the apparatus, is represented by a dotted line O. A targetgrating T and diffracted rays are illustrated in more detail in FIG.5(b).

The same reference numbers are used for components described already inthe FIG. 4 apparatus. The illumination path is labeled IP as before. Thereference path RP is omitted, for clarity. Compared with that apparatus,a second beam splitter 17 divides the collection path into two branches.In a first measurement branch, detector 19 records a scatter spectrum ordiffraction spectrum of the target exactly as described above. Thisdetector 19 may be referred to as the pupil image detector.

In the second measurement branch, imaging optical system 22 forms animage of the target on the substrate W on sensor 23 (e.g. a CCD or CMOSsensor). An aperture stop 21 is provided in a plane that is in thecollection path in a plane conjugate to the pupil-plane (it may also becalled a pupil stop). Aperture stop 21 can take different forms, just asthe illumination aperture can take different forms. Typically, aperturestop 21 functions to block the zeroth order diffracted beam so that theimage of the target formed on sensor 23 is formed only from the firstorder beam(s). This is the so-called dark field image, equivalent todark field microscopy. The images captured by sensors 19 and 23 areoutput to image processor and controller PU, the function of which willdepend on the particular type of measurements being performed.

In the illumination path in this example, additional optics are shownsuch that a field stop 13′ can be placed in a plane conjugate with theplane of the target and the image sensor 23. This plane may be referredto as a field plane, or conjugate image plane, and has the property thateach spatial position across the field plane corresponds to a positionacross the target. This field stop may be used for example to shape theillumination spot for a particular purpose, or simply to avoidilluminating features that are within the field of view of the apparatusbut not part of the target of interest. The following drawings anddiscussion refer, by way of example, to techniques for implementation ofthe function of aperture device 13, but the present disclosure alsoencompasses use of the same techniques to implement the function offield stop 13′.

As shown in more detail in FIG. 5(b), target grating T is placed withsubstrate W normal to the optical axis O of objective lens 16. In thecase of an off-axis illumination profile, A ray of illumination Iimpinging on grating T from an angle off the axis O gives rise to azeroth order ray (solid line 0) and two first order rays (dot-chain line+1 and double dot-chain line −1). It should be remembered that with anoverfilled small target grating, these rays are just one of manyparallel rays covering the area of the substrate including metrologytarget grating T and other features. Since the aperture in plate 13 hasa finite width (necessary to admit a useful quantity of light, theincident rays I will in fact occupy a range of angles, and thediffracted rays 0 and +1/−1 will be spread out somewhat. According tothe point spread function of a small target, each order +1 and −1 willbe further spread over a range of angles, not a single ideal ray asshown.

Different modes of illumination are possible by using differentapertures. Apertures 13N (north′) and 13S (south′) each provide off-axisillumination from a specific narrow range of angles only. Returning toFIG. 5(a), this is illustrated by designating diametrically oppositeportions of the annular aperture as north (N) and south (S). The +1diffracted rays from the north portion of the cone of illumination,which are labeled +1(13N), enter the objective lens 16, and so do the −1diffracted rays from the south portion of the cone (labeled −1(13S)). Asdescribed in the prior applications mentioned in the introduction, usingthe dark-field imaging sensor 23 while switching between apertures 13N,13S of this type is one way of obtaining asymmetry measurements frommultiple small targets. Aperture stop 21 a can be used to block thezeroth order radiation when using off-axis illumination.

While off-axis illumination is shown, on-axis illumination of thetargets may instead be used and an aperture stop with an off-axisaperture is used to pass substantially only one first order ofdiffracted light to the sensor. In one example, prisms 21 b are used inplace of aperture stop 21 which have the effect of diverting the +1 and−1 orders to different locations on sensor 23 so that they can bedetected and compared without making two images. This technique, isdisclosed in the above-mentioned published patent applicationUS2011102753A1, the contents of which are hereby incorporated byreference. 2nd, 3rd and higher order beams (not shown in FIG. 5) can beused in measurements, instead of or in addition to the first orderbeams.

When monitoring a lithographic process, it is desirable to monitor focusof the lithography beam on the substrate. One known method ofdetermining the focus setting from a printed structure is by measuringthe critical dimension (CD) of the printed structure. CD is a measure ofthe smallest feature (e.g., line width of an element). The printedstructure may be a target, such as a line-space grating, formedspecifically for focus monitoring. It is known that CD usually displays2nd order response to focus, forming what is known as a “Bossung curve”on a plot of CD (y-axis) against focus (x-axis). A Bossung curve is asubstantially symmetrical curve which is substantially symmetricalaround a peak representing the best focus. The Bossung curve may besubstantially parabolic in shape. There are several drawbacks to thisapproach. One drawback is that the method shows low sensitivity nearbest focus (due to the parabolic shape of the curve). Another drawbackis that the method is insensitive to the sign of any defocus (as thecurve is largely symmetrical around best focus). Also this method issensitive to inter alia dose and process variation (crosstalk).

To address these issues, diffraction based focus (DBF) was devised.Diffraction based focus may use target forming features on the reticlewhich print targets having a degree of asymmetry which is dependent onthe focus setting during printing. This degree of asymmetry can then bemeasured using a scatterometery based inspection method, for example bymeasuring the intensity asymmetry between the intensities of +1st and−1st order radiation diffracted from the target, to obtain a measure ofthe focus setting.

FIG. 6 illustrates DBF target forming design 615 configured fordiffraction based focus measurements. It comprises plural DBF structures620, each of which comprises high resolution substructures 625. The highresolution substructures 625 on top of a base pitch creates anasymmetric resist profile for each DBF structure 620, with the degree ofasymmetry being dependent upon focus. Consequently a metrology tool canmeasure the degree of asymmetry from a target formed using DBF targetforming design 615 and translate this into the scanner focus.

While the DBF target forming design 615 enables diffraction based focusmeasurements, it is not suitable for use in all situations. EUV resistfilm thicknesses are significantly lower than those used in immersionlithography, which makes it difficult to extract accurate asymmetryinformation from the asymmetric profile of the structures forming partof a target. In addition such structures may not comply with the strictdesign constraints applicable to certain product structures. During thechip making process all features on the reticle must print and stand upto subsequent processing steps. Semiconductor manufacturers use designrules as a means to restrict the feature designs to ensure the printedfeatures conform to their process requirements. An example of such adesign rule relates to the allowable size of structures or pitches.Another example design rule relates to pattern density, which mayrestrict the density of a resulting resist pattern to be within aparticular range.

It is therefore proposed to monitor focus using at least a first targetand a second target which have been formed with a best focus offset dfbetween the two targets. As before, the focus response with ameasurement value for a target parameter (for example CD or othermeasurements as will be described below) takes the form of a Bossungcurve for each of the first and second targets. Focus is a function of afirst measurement value of a target parameter obtained from measurementof the first target and a second measurement value of a target parameterobtained from measurement of the second target. Therefore, it isproposed that measured values of a parameter from the first and secondtargets be obtained and a value for focus derived from these measuredvalues. A specific example of how focus may be derived is describedbelow, with reference to FIG. 7. However, the skilled person willrealise that there are many alternative methods that allow focus to beextracted from the measured values obtained from the first and secondtargets. While the description below specifically discusses using adifference of the two measurement values (whether they be intensityvalues or otherwise), other mathematical operations and methods may beused to extract a focus value. For example, it is possible to divide oneof the measurement values (from one of the first and second targets)into the other measurement value (from the other of the first and secondtargets).

FIG. 7(a) shows a plot of a target parameter Pt against focus f for boththe first target and second target. It shows a first Bossung curve 700corresponding to the first target and a second Bossung curve 710corresponding to the second target. Also shown is the best focus offsetdf (the focus offset between the two peaks of Bossung curves 700, 710).Where the targets overlap 720 is the focus range through which adifference of the target parameter for the first target and the targetparameter for the second target has an essentially linear relationshipwith focus. This is illustrated in FIG. 7(b), which is a plot of thisdifference Pt2−Pt1 (where Pt1 is the target parameter for the firsttarget and Pt2 is the target parameter for the second target) againstfocus. As can be seen, the relationship 740 is linear. The Pt2−Pt1metric may be sensitive to crosstalk, e.g. by dose and/or process. Amore robust metric may be Pt2−Pt1/PtAV, where PtAV, is the average ofPt2 and Pt1. The relationship 750 (dotted) of Pt2−Pt1/PtAV with focus isalso shown in FIG. 7(b). This relationship is still sufficiently linearwhile being more robust against crosstalk.

In the specific example illustrated, the slope of relationship 740 orrelationship 750 can be described by 2*df*a where df is the best focusoffset and a is the Bossung curvature. Accordingly, focus can beobtained from the following equation (Equation 1):

$f = \frac{P_{t\; 2} - P_{t\; 1}}{2 \cdot {df} \cdot a \cdot P_{tAV}}$where P_(tAV) in the denominator is optional.

To increase focus sensitivity, it is possible to increase the best focusoffset, thereby increasing the slope of relationship 740 or relationship750.

In the above discussion, it should be appreciated that any targetparameter Pt can be used provided it has a Bossung curve response withfocus. While CD may be used, a new diffraction based focus methodology,is proposed which has significant advantages over prior methods. Thismethod comprises using intensity signals obtained from diffractionorders of radiation scattered by the first target and second target todetermine focus. In particular, it is proposed to use intensity valuesof a single diffraction order from each of the first and second targetsto determine focus. The proposed method may use intensity values fromcorresponding diffraction orders of the first and second targets. Forexample, the intensity values could be that of the +1st (or −1st)diffraction orders from the first and second targets. Alternatively, orin combination, the intensity values could be that of the zerothdiffraction orders from the first and second targets.

In a specific example of such a method, it is proposed to use thedifference dI in the measured intensity of a diffraction order ofradiation scattered by the first target and the measured intensity ofthe corresponding diffraction order of radiation scattered by the secondtarget. This difference is hereafter referred to as the dI metric.However, the dI metric may be any metric derived from diffractionintensity values from the first and second targets (e.g., by dividingone of the intensity values into the other).

In an embodiment, the dI metric may be the difference in correspondingfirst diffraction orders (or higher diffraction orders), e.g., the +1stdiffraction order of radiation scattered by the first target and the+1st diffraction order of radiation scattered by the second target(clearly minus orders could equally be used). In another embodiment, thedI metric may comprise the difference between intensity measurements ofzeroth orders from the first and second targets.

As before, the dI metric may be divided by an average of the intensitymeasurements Iav, to reduce the effect of crosstalk. However, theBossung curvature for the dI metric is only weakly dependent on dose,such that the dI metric may already exhibit a sufficiently low dosecrosstalk.

Using the dI metric in this way provides good signal strength andsignal-to noise response, even when the targets comprise shallowgratings (e.g., for use in EUV lithography).

As mentioned above, the dI metric may comprise the difference of thezeroth orders of radiation scattered by the first and second targets. Inthis way, targets with smaller pitches can be used. Consequently, targetpitch for the first and second targets can be chosen to agree with anycustomer design rules. Also, smaller target pitches means that theoverall target size can be reduced. Multiple pitches are also possible.Using the zeroth order radiation means that the diffracted radiationdoes not need to be captured, and the dI metric describes differences inlight absorbed by targets having a relative best focus offset.Measurement of zeroth orders may also increase signal strength andsignal-noise characteristics.

Where first order diffracted radiation is used, and since only a singlefirst order is required per measurement, the pitch required to use firstorder radiation light is reduced to λ/2 (where λ is the detectionwavelength) in the limit of numerical aperture NA=1. At present thislimit is λ. This will mean that the linear target dimension can bereduced by a factor of 2 and the real estate by a factor of 4.

The best focus offset between the first target and second target can beintroduced in a number of ways. In one embodiment, the lithographicapparatus used to print the targets may have a deliberate, controlledastigmatism. The astigmatism may be introduced to the projection opticsvia a number of manipulators included within the projection optics. Theprojection lenses in many lithographic apparatuses enable a sufficientlylarge astigmatism offset to create a best focus offset, without unwantedwavefront effects. In an embodiment, the astigmatism may introduce abest focus offset between horizontal and vertical features. To exploitthis, the first and second targets may comprise respectively ahorizontal grating and a vertical grating (or vice versa).

In an embodiment, the best focus offset can be introduced by the reticle(also referred to as a patterning device or mask). It is proposed tohave pairs of targets (e.g., line-space gratings) incorporated on areticle. The reticle may contain locations approximately the size of atarget (for example 20×20 μm, 8×8 μm or 5×5 μm) and a border zone wherethe substrate is etched to a depth d. One of the first and secondtargets pair is deposited at normal mask level, the other at a(preferably adjacent) etched position.

FIG. 8 illustrates a number of alternative reticle arrangements forachieving such an arrangement. FIG. 8a shows, in cross section, areference target which may be used to print one of said first target andsaid second target. This is a conventional target feature on a reticle,comprising radiation blocking structures 800 on a transparent reticlesubstrate 810. The reticle may be of any structure or material. Forexample, the transparent reticle substrate 810 may comprise quartzglass, and the radiation blocking structures 800 may comprise chromium,molybdenum silicide (any opacity) or tantalum boron nitride.

It is proposed that this reference target is used with one of the targetarrangements of FIG. 8(b), 8(c) or 8(d). However, any combination of two(or more) of any of the targets shown in FIG. 8 may be used providedthey result in a relative best focus offset.

FIG. 8(b) shows a transparent reticle substrate 810 which, in the regionof the target, has been etched to a depth d before the addition ofradiation blocking structures 800. Such an arrangement provides a simpletarget, but manufacture is complicated by not being able to do this inthe “mask shop”. FIG. 8(c) shows an arrangement similar to that of FIG.8(a), but where the reticle substrate 810 has been etched through to adepth d after deposition of the radiation blocking structures 800. FIG.8(d) shows an arrangement where the radiation blocking structures 800are topped with additional metal (e.g., chromium) caps 820. This issimilar to a TIS (transmission image sensor) target. Such an arrangementis not possible in EUV lithography.

In the etched examples above, the depth d may be for example 0.1 μm ormore, more specifically in the region of 0.1 μm to 5 μm, or 0.5 μm to 5μm, and for example 0.5 μm to 3 μm. In an embodiment, depth d may be inthe region of 1 μm.

FIG. 9 illustrates a further reticle arrangement for achieving a bestfocus offset in a first (reference) target. This arrangement is suited(by way of example) for OMOG (Opaque MoSi on Glass) and for attPSM(attenuated phase-shifting mask) reticle types. In particular, the trendfor attPSM reticles is for a reduction in the Cr thickness. This willreduce the Cr topping effect of FIG. 8(d) embodiment described above.The reticle production comprises depositing one (or more) extra absorberstacks are on the blank; where the blank comprises the reticle substrateon which is deposited a single absorber stack. The absorber stack maycomprise an opaque layer (e.g., a MoSi layer) topped with a metal layer(e.g., a Cr layer).

FIG. 9(a) shows the new reticle blank. It comprises a reticle substrate910 topped with two absorber stacks. A first absorber stack comprisesfirst layer 920 a (e.g., a MoSi layer) and second layer 920 b (e.g., aCr layer). The second absorber layer also comprises two layers: thirdlayer 920 c (e.g., a MoSi layer) and fourth layer 920 d (e.g., a Crlayer).

FIG. 9(b) shows the final reticle arrangement. It shows first target 930and second target 940. First target 930 is conventional in that itcomprises a single opaque layer (e.g. formed from first layer material920 a) of blocking structures 900. The second target 940 comprisesblocking structures 950, each having three layers: a first layer 950 a,a second layer 950 b and a third layer 950 c, formed respectively fromfirst layer material 920 a, second layer material 920 b and third layermaterial 920 c. The fourth layer 920 d is removed completely.

The process for producing such a reticle may comprise the followingsteps:

Deposit one or more extra absorber stacks on the blank (this may beperformed by the provider of the blank);

Etch the target layer (containing the targets 930, 940) through bothstacks to the depth of substrate 910.

Remove the extra stack for the first target and its corresponding layer.This layer contains the product and the reference targets (the firsttargets), but not the second targets. Resist covers the second targetsduring this step;

Etch the reference layer in a conventional manner. Resist covers thesecond targets during this step

A further method for obtaining two targets with a best focus offsetbetween them comprises providing a first target comprising a line-spacetarget with a focus insensitive side wall angle (SWA), such that the SWAof the individual structures of the first target is focus insensitive,and a second target with focus sensitive SWA. The second target maycomprise a segmented line, the segmentation being sub-resolution withrespect to the lithography apparatus.

FIG. 10 illustrates a reticle arrangement for producing such first andsecond targets. The first target 1000 (shown in part) comprises aline-space target having structures 1010 which produce correspondingtarget structures on a substrate with an SWA that is focus insensitive.In an embodiment, the SWA is small (i.e., close to vertical). The secondtarget 1020 (shown in part) comprises a line-space target havingsegmented line structures 1030. The segmented line structures 1030comprise high resolution substructures 1040, which may be similar tohigh resolution substructures 625 of FIG. 6. The second target 1020 issuch that the resulting target exposed on a substrate has a focusdependent SWA.

The first target 1000 and second target 1020 each have a targetparameter response with focus which describe Bossung curves having abest focus offset, similar to the response illustrated in FIG. 7. Thisbest focus offset is as a result of the focus dependent SWA of only oneof the targets. SWA varies linearly with focus, which causes the shiftin the Bossung peak. By this method, an asymmetric target (such as thatillustrated in FIG. 6) can be separated into two separate symmetrictargets having similar performance. This enables more efficient targetselection and the use of the full pitch for parameter values.

An advantage of introducing the best focus offset in the reticle (ratherthan via astigmatism in the projection optics) is this allows bothon-product and off-product focus monitoring. The requirement ofastigmatism in the projection optics means that such methods can only beused for off-product monitoring.

FIG. 11 is a flowchart of the steps of a method for monitoring a focusparameter during a lithographic process according to an exemplaryembodiment. The steps are as follows, and are then described in greaterdetail thereafter:

1100—Start.

1110—Print first and second targets with a relative best focus offset;

1120—Perform first measurement from inspection of the first target toobtain first measurement value;

1130—Perform second measurement from inspection of second target toobtain second measurement value;

1140—Calculate focus from difference of first measurement value andsecond measurement value;

1150—Use calculated focus measurement in focus setting for subsequentexposures.

1160—End.

At step 1110, first and second targets (at least) are printed with arelative best focus offset as already described. The relative best focusoffset may be introduced, for example, via a relative depth offset onthe reticle between the target forming structures which form the firstand second targets. Alternatively, relative best focus offset may beintroduced via astigmatism in the projection optics of the lithographicsystem. By way of a further alternative example, the reticle arrangementillustrated in FIG. 10 may be used. Other methods of introducing arelative best focus offset between two targets are also possible and areenvisaged within the scope of this disclosure.

At step 1120, a first measurement is performed from inspection of thefirst target to obtain a first measurement value for a target parameter.In an embodiment, this first measurement may be of the intensity (orrelated parameter) of one of the diffraction orders of radiationscattered by the first target. This first measurement may be obtainedusing any of the scatterometer devices described herein, for example. Itis also contemplated within the scope of this disclosure that the firstmeasurement be a CD measurement (whether obtained using a scatterometer,scanning electron microscope or other suitable device) or any othermeasurement of a parameter which has a Bossung curve relationship withfocus.

At step 1130, a second measurement is performed from inspection of thesecond target to obtain a second measurement value for a targetparameter. This second measurement should be performed using the samemethod as the first measurement. Where the first measurement is of theintensity (or related parameter) of one of the diffraction orders ofradiation scattered by the first target, the second measurement shouldbe of the same diffraction order of radiation scattered by the secondtarget. The diffraction order may be either of the first diffractionorders or the zeroth diffraction order. However, higher diffractionorders can also be used and are within the scope of the disclosure.Further measurements may be made if there are more than two targetsprinted. These additional targets may each comprise best focus offsetsthat are different to that of said first target and/or said secondtarget.

It should be appreciated that step 1120 and step 1130 may be performedas a single step such that the first measurement value and secondmeasurement value are obtained in a single acquisition. In addition,where there are more than two targets being measured, all the targetsmay be measured in a single acquisition to obtain a corresponding numberof measurement values. In a specific example, a measurement device, suchas that illustrated in FIG. 5, can be used to measure a composite targetcomprising multiple individual targets (individual periodic structuresor gratings). The gratings of the composite target may be positionedclosely together so that they will all be within an image field ormeasurement spot formed by the illumination beam of the metrologyapparatus. In this way, the gratings can be all simultaneouslyilluminated and simultaneously imaged on the detector. These images canthen be processed to identify the separate images of the gratings. Thiscan be done by pattern matching techniques, so that the images do nothave to be aligned very precisely at a specific location within thesensor frame. Once the separate images of the gratings have beenidentified, the intensities of those individual images can be measured,e.g., by averaging or summing selected pixel intensity values within theidentified areas. In another embodiment, the first and second targetsmay be comprised within a composite target, but measured separately intwo separate acquisitions.

At step 1140, the focus is calculated from the first and secondmeasurement values, for example from the difference of the first andsecond measurement values. This calculation can be performed usingEquation 1 or other suitable equation or method.

At step 1150, the calculated focus can then be used in focus parametermonitoring during subsequent lithographic processes, so as to maintainfocus accuracy and consistency during exposure.

The above discussion describes methods for determining focus. However,also disclosed is a method for measurement of dose. Current diffractionbased dose metrology is based on simulation of the diffraction patternof a parametrized resist pattern. The parameters are then adjusted sothat the resulting zero order diffraction efficiency, notably itsangular dependency, agrees with measurements. Line/space (LS) targetswith CD and pitch in a range of interest are used. This method isreferred to as CD reconstruction (CDR), and depends on the correctnessof the parametrized model. The model is necessarily a schematicapproximation to the resist pattern with a limited number of parameters.The model requires knowledge of the geometry and optical parameters ofthe stack. This is commonly proprietary information and thereforedifficult to obtain, and may be inaccurate.

Therefore, a simpler method for determining dose is proposed, whichcomprises forming a first line-space target and a second line-spacetarget having resist patterns with inverse duty cycles, or in pairs ofresist patterns with matched properties such that cross talk, bothoffset and scaling, is minimised. In an embodiment, the first and secondtargets each have the same pitch but, where the first target is a linetarget, the second target will be a corresponding space target, suchthat the width of the lines of the first target is equal to the width ofthe spaces of the second target. FIG. 12 illustrates such a targetarrangement having a first target 1200 with individual resist features1210 having a CD a, and a second target 1220 with individual resistfeatures 1230 having a CD b, each target having the same pitch.

FIG. 13 shows a graph 1310 of Resist (line) CD against first orderintensity I for a line space target of pitch 600 nm. It can be seen thatthe measured intensity for corresponding line and space targets (e.g., afirst target with resist CD of 150 nm and a second target with resist CDof 450 nm) should be approximately the same. However resist CD isdependent on dose, such that an increase in dose results in a decreasein resist CD and vice versa. Consequently, an increase in dose (forexample), will cause a decrease in resist CD for both the first targetand second target. This will cause a decrease in the first orderintensity for the first target and an increase in the first orderintensity for the second target. The dose sensitivity for the first andsecond targets can therefore be seen to be opposite. The difference ofthe intensity measurements for the first and second targets cantherefore be a target parameter used as a dose metric.

It can be noted that any process variation, to a certain extent, mayhave the same effect as dose variation. Process variations such as postexposure bake (PEB) and secondary electron blur (SEB) variation, as wellas bottom anti-reflective coating (BARC) and resist thickness variation,alter the exposure intensity in the thin resist film. However, BARC andresist thickness variation affect the measured diffraction intensity:thicker BARC and resist results in an increase in the measured firstorder intensity. This effect does not depend on CD and the resultingintensity change is equal for both targets. If the targets have amatching nominal first order intensity response, then any crosstalkinduced signal offset will cancel when taking the difference ofmeasurements from the two targets. Any crosstalk induced scaling will beequal for the signal difference and the signal average. Therefore it isexpected that the ratio of difference and average measured intensitieswill be robust against process crosstalk, but sensitive to doseincluding process induced dose effects.

It is proposed to measure dose calibration curves as a function offocus. The actual dose can be inferred from the dose calibration,assuming that focus is known. Focus can be determined using any of themethods disclosed herein, by earlier diffraction based focus methods(e.g., using structures of a form shown in FIG. 6), or any othersuitable method.

While the first and second targets are described as line-space targets,they can comprise any dose dependent resist targets which producesuitable first order responses. In an embodiment, a correction may bedetermined to account for any difference in response to process induceddose effects between the target and product.

The proposed method enables first order intensity measurements to beused to determine dose without the need for model simulation orprediction. The method suppresses sensitivity to process variations onthe target performance. The impact of illumination and dose conditionson target performance is small. The method is sensitive to scanner doseand dose-like process effects. The use of first order signals are moreaccurate than using zeroth order signals as they have better shot noiseperformance.

The targets are described above as line-space grating targets, as theseare simple to produce and measure. However, the targets may comprise anystructure which results in a Bossung curve response between a measurabletarget parameter and focus. For example, the targets may comprisecombined horizontal and vertical line-space gratings, forming a “contacthole” arrangement. Such a target may enable more diffraction orders tobe captured. The target arrangements may comprise more than two targets.Consequently, the methods described herein may comprise performingmeasurements on more than two targets.

In some circumstances, design rules are imposed which result inconstraints on certain parameters of reticle features. An example ofsuch design rules are the provision of design grids which have (andtherefore impose on the target) a fixed pitch and/or CD for a line-spacetarget. It may be that many of the targets described herein wouldinfringe such design rules.

By way of specific example, a design grid based design rule may impose atarget pitch of 100 nm and a CD of 40 nm; that is lines can only beformed with a CD of 40 nm and on a grid having a 100 nm pitch in thedirection of the line-space grating. However, it may be desirable that atarget actually has a pitch of 600 nm, such that first order signals canbe detected and measured. It is proposed that such a line-type targetcan be obtained by providing 1 or 2 such lines on corresponding gridlocations in a row of such grids. Each grid therefore will define asingle target feature. An analogous space-type target can be obtained byproviding 4 or 5 such lines on corresponding grid locations in a row ofsuch grids.

It is desirable to image these patterns such that they have the designeddimensions and a sufficiently large depth of focus on the substrate.Therefore, the lines may be biased (e.g., in a manner similar to opticalproximity correction methods) and (for example 20 nm) assist featuresmay optionally be placed on the empty locations on each grid. In thisway, the target may be made more stable and more symmetrical (e.g., interms of SWA).

FIGS. 14(a) to 14(f) each comprise an example of possible targetfeatures based upon a grid. In particular, where the CD is 40 nm andgrid pitch 100 nm as per the example previously given, the 2 lineexample of FIG. 14(e) and 4 line example of FIG. 14(c) emulate targetfeatures for respectively line and space targets having a CD of about100-150 nm and pitch of 600 nm.

A method of manufacturing devices using the lithographic process can beimproved by providing an inspection apparatus as disclosed herein, usingit to measure processed substrates to measure parameters of performanceof the lithographic process, and adjusting parameters of the process(particularly focus) to improve or maintain performance of thelithographic process for the processing of subsequent substrates.

It should be understood that the particular parameters used in the aboveexamples are not the only ones that may be defined. Additional and/oralternative parameters can be used in a real design process, accordingto limitations of the lithographic apparatus and the inspectionapparatus to be used for the metrology. While the target structuresdescribed above are metrology targets specifically designed and formedfor the purposes of measurement, in other embodiments, properties may bemeasured on targets which are functional parts of devices formed on thesubstrate. Many devices have regular, grating-like structures. The terms‘target grating’ and ‘target structure’ as used herein do not requirethat the structure has been provided specifically for the measurementbeing performed.

In association with the physical grating structures of the targets asrealized on substrates and patterning devices, an embodiment may includea computer program containing one or more sequences of machine-readableinstructions describing a methods of designing metrology recipes and/orcontrolling the inspection apparatus to implement the illumination modesand other aspects of those metrology recipes. This computer program maybe executed for example in a separate computer system employed for thedesign/control process. Alternatively, the design process may be whollyor partly performed within unit PU in the apparatus of FIG. 3, 4 or 5and/or the control unit LACU of FIG. 2. There may also be provided adata storage medium (e.g., semiconductor memory, magnetic or opticaldisk) having such a computer program stored therein.

Further embodiments according to the invention are provided in belownumbered clauses:

-   -   1. A method of monitoring a focus parameter during a        lithographic process, said method comprising:    -   acquiring a first measurement value, said first measurement        value having been obtained from inspection of a first target;    -   acquiring a second measurement value, said second measurement        value having been obtained from inspection of a second target,    -   wherein said first target and second target have been exposed        with a relative best focus offset;    -   determining the focus parameter from said first measurement        value and said second measurement value.    -   2. A method according to clause 1 wherein said first measurement        value has been obtained from a first measurement of radiation        scattered from said first target and said second measurement        value has been obtained from a second measurement of radiation        scattered from said second target.    -   3. A method according to clause 2 wherein said first measurement        is an intensity measurement of a diffraction order of radiation        scattered from said first target and said second measurement is        an intensity measurement of a corresponding diffraction order of        radiation scattered from said second target.    -   4. A method according to clause 3 wherein said diffraction order        is the zeroth diffraction order.    -   5. A method according to clause 3 wherein said diffraction order        is a non-zeroth diffraction order.    -   6. A method according to any preceding clause comprising the        steps of performing said first measurement to obtain said first        measurement value and performing said second measurement to        obtain said second measurement value.    -   7. A method according to any of clauses 1 to 5 comprising        inspecting at least said first target and said second target in        a single measurement to obtain said first measurement value and        said second measurement value.    -   8. A method according to any preceding clause wherein said step        of determining the focus parameter comprises determining the        focus parameter from the difference of said first measurement        value and said second measurement value.    -   9. A method according to clause 8 wherein said difference of        said first measurement value and said second measurement value        is divided by an average of said first measurement value and        said second measurement value.    -   10. A method according to clause 8 or 9 wherein the variation of        said first measurement value with focus and the variation of        said second measurement value with focus each define        corresponding Bossung curves having a relative focus offset, and        the variation of said difference of said first measurement value        and said second measurement value with focus is substantially        linear.    -   11. A method according to clause 10 wherein the variation of        said difference of said first measurement value and said second        measurement value with focus is defined by a line having a slope        dependent upon the curvature of said Bossung curves and the        relative best focus offset.    -   12. A method according to any preceding clause wherein the        relative best focus offset results from astigmatism in the        lithographic apparatus during formation of said first target and        second target.    -   13. A method according to clause 12 wherein said astigmatism        results in a relative best focus offset between horizontal and        vertical structures, and wherein said first target comprises        substantially horizontal structures and said second target        comprises substantially vertical structures.    -   14. A method according to any of clauses 1 to 12 wherein the        relative best focus offset is resultant from the patterning        device which defines said first target and second target.    -   15. A method according to clause 14 wherein there is an offset        in the depth of the patterns in said patterning device which        define said first target and second target.    -   16. A method according to clause 14 or 15 wherein one of the        patterns for defining one said first target or said second        target comprises line features which are taller, in a direction        transverse to the plane of the target when compared to the other        of said first target or said second target.    -   17. A method according to clause 16, having at least one        additional layer of absorber material.    -   18. A method according to clause 17 wherein the additional layer        comprises one of: a metal, molybdenum silicide or tantalum boron        nitride.    -   19. A method according to clause 17 wherein the additional layer        comprises an additional stack of absorber material.    -   20. A method according to clause 14 wherein said first target        comprises line features having a substantially focus independent        side wall angle and said second target comprises line features        having a focus dependent side wall angle.    -   21. A method according to clause 20 wherein each of said first        target and said second target comprise line-space grating        structures, and wherein the pattern on the patterning device        which defines said second targets comprise segmented line        features, each segmented line feature having a pitch below the        imaging resolution of the lithographic process.    -   22. A method according to any preceding clause comprising        forming said first target and said second target on a substrate        with said relative best focus offset.    -   23. A method according to any preceding clause wherein said        first target and said second target each comprise line-space        grating structures.    -   24. A method according to any preceding clauses wherein design        rules constrain target features to a set critical dimension and        on a grid of set pitch, wherein said first target and said        second target are each formed from rows of said grids such that        each line feature of said first target and said second target        are formed from one, or plural adjacent, corresponding target        features formed on said grids.    -   25. A method according to any preceding clause wherein patterns        defining said first target and said second target do not        comprise features which purposely introduce focus dependent        asymmetry in said first target and said second target.    -   26. A method according to any preceding clause comprising:    -   making additional measurements of additional targets, said        additional targets being additional to said first target and        said second target, each of said additional targets having a        different best focus to that of said first target and/or said        second target; and    -   using said additional measurements in said step of determining        the focus parameter.    -   27. A method of monitoring a dose parameter during a        lithographic process, said method comprising:    -   acquiring a first measurement value, said first measurement        value having been obtained from inspection of a first target;    -   acquiring a second measurement value, said second measurement        value having been obtained from inspection of a second target,    -   determining the dose parameter from said first measurement value        and said second measurement value;    -   wherein said first and second targets comprise corresponding        line and space targets having the same pitch and inverse duty        cycles.    -   28. A method according to clause 26 or 27 wherein said step of        determining the dose parameter comprises determining the dose        parameter from the difference of said first measurement value        and said second measurement value.    -   29. A method according to any of clauses 26 to 28 wherein said        step of determining the dose parameter comprises:    -   determining a focus parameter of said lithographic process; and    -   referring to a dose calibration curve corresponding to said        determined focus parameter.    -   30. A method according to clause 29 wherein said step of        determining the dose parameter comprises performing the method        of any of clauses 1 to 26.    -   31. A metrology apparatus for measuring a parameter of a        lithographic process, the metrology apparatus being operable to        perform the method of any of clauses 1 to 30.    -   32. A metrology apparatus according to clause 31 comprising:    -   a support for said substrate having a plurality of targets        thereon;    -   an optical system for measuring each target; and    -   a processor.    -   33. A lithographic system comprising:    -   a lithographic apparatus comprising:    -   an illumination optical system arranged to illuminate a pattern;    -   a projection optical system arranged to project an image of the        pattern onto a substrate; and    -   a metrology apparatus according to clause 31 or 32,    -   wherein the lithographic apparatus is arranged to use the        determined focus parameter and/or dose parameter calculated by        the metrology apparatus in applying the pattern to further        substrates.    -   34. A computer program comprising processor readable        instructions which, when run on suitable processor controlled        apparatus, cause the processor controlled apparatus to perform        the method of any one of clauses 1 to 26.    -   35. A computer program carrier comprising the computer program        of clause 34.    -   36. A method of manufacturing devices wherein a device pattern        is applied to a series of substrates using a lithographic        process, the method including:        -   using the method of any of clauses 1 to 26 to monitor said            focus parameter, and        -   controlling the lithographic process for later substrates in            accordance with the determined focus parameter.    -   37. A method of manufacturing devices wherein a device pattern        is applied to a series of substrates using a lithographic        process, the method including:        -   using the method of any of clauses 27 to 30 to monitor said            dose parameter, and        -   controlling the lithographic process for later substrates in            accordance with the determined dose parameter.    -   38. A patterning device configured to pattern a beam of        radiation in a lithographic process according to a desired        pattern, said patterning device comprising first features for        forming a first target on a substrate during the lithographic        process, and second features for forming a second target on the        substrate during the lithographic process; wherein said second        features are taller than said first features, in a direction        transverse to the plane of the target and such that said first        target and said second target have a relative best focus offset.    -   39. A patterning device according to clause 38 wherein said        first features and second features are deposited on a reticle        substrate and said second features are deposited on a section of        the reticle substrate which has been etched to a different        level, in a direction transverse to the plane of the target,        compared to a section of the reticle substrate on which said        first features have been deposited.    -   40. A patterning device according to clause 38 or 39 wherein        said second features comprise one or more additional layers than        said first features.    -   41. A patterning device according to clause 40 wherein the one        or more additional layers comprises a layer of absorber        material.    -   42. A patterning device according to clause 41 wherein the        additional layer of absorber material comprises one of: a metal,        molybdenum silicide or tantalum boron nitride.    -   43. A patterning device according to clause 40 wherein the one        or more additional layers comprises an additional stack of        absorber material, said additional stack of absorber material        comprising a first layer of absorber material and a second layer        of absorber material.    -   44. A patterning device according to clause 43 wherein the first        layer of absorber material comprises molybdenum silicide or        tantalum boron nitride, and said second layer of absorber        material comprises a metal.    -   45. A patterning device configured to pattern a beam of        radiation in a lithographic process according to a desired        pattern, said patterning device comprising first features for        forming a first target on a substrate during the lithographic        process, and second features for forming a second target on the        substrate during the lithographic process; wherein said first        features are configured to form said first target with line        features having a substantially focus independent side wall        angle and said second features are configured to form said        second target with line features having a focus dependent side        wall angle.    -   46. A patterning device according to clause 45 wherein each of        said first target and said second target comprise line-space        grating structures, and wherein the second features comprise        segmented line features, each segmented line feature having a        pitch below the imaging resolution of the lithographic process.    -   47. A patterning device configured to pattern a beam of        radiation in a lithographic process according to a desired        pattern, said patterning device comprising first features for        forming a first target on a substrate during the lithographic        process, and second features for forming a second target on a        substrate during the lithographic process; wherein design rules        constrain target features to a set critical dimension and on a        grid of set pitch, wherein said first target and said second        target are each formed from rows of said grids such that each of        said first features and said second features are formed from        one, or plural adjacent, corresponding target features formed on        said grids.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.,having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g., having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description by example, and not oflimitation, such that the terminology or phraseology of the presentspecification is to be interpreted by the skilled artisan in light ofthe teachings and guidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

The invention claimed is:
 1. A method of monitoring a focus parameterduring a lithographic process, the method comprising: acquiring a firstmeasurement value, the first measurement value having been obtained frominspection of a first target; acquiring a second measurement value, thesecond measurement value having been obtained from inspection of asecond target, wherein the first and second targets have been exposedwith a relative best focus offset; and determining the focus parameterfrom a difference of the first and second measurement values, whereinthe difference of the first and second measurement values is divided byan average of the first and second measurement values, wherein the firstand second measurement values comprise intensity values of a diffractionorder of radiation scattered by the first and second targets,respectively.
 2. The method of claim 1, wherein the relative best focusoffset results from astigmatism in the lithographic apparatus duringformation of the first and second targets.
 3. The method of claim 1,further comprising: forming the first and second targets on a substratewith the relative best focus offset.
 4. The method of claim 1, whereinthe first and second targets each comprise line-space gratingstructures.
 5. The method of claim 1, wherein: design rules constraintarget features to a set critical dimension, and wherein on a grid ofset pitch, the first and second targets are each formed from rows of thegrid, such that each line feature of the first target and second targetsare formed from one or more adjacent, corresponding target featuresformed on the grid.
 6. The method of claim 1, wherein patterns definingthe first and second targets do not comprise features which purposelyintroduce focus dependent asymmetry in the first and second targets. 7.The method of claim 1, further comprising: acquiring additionalmeasurements of additional targets, the additional targets beingadditional to the first and second targets, and each of the additionaltargets having a different best focus to that of the first target and/orthe second target; and using the additional measurements in the step ofdetermining the focus parameter.
 8. The method of claim 1, wherein thediffraction order comprises a first diffraction order of radiationscattered from the first and second targets.
 9. The method of claim 1,wherein the diffraction order comprises a zeroth diffraction order ofradiation from the first and second targets.
 10. The method of claim 1,wherein the first and second targets comprise corresponding line andspace targets having the same pitch and inverse duty cycles.
 11. Amethod of monitoring a dose parameter during a lithographic process, themethod comprising: acquiring a first measurement value, the firstmeasurement value having been obtained from inspection of a firsttarget; acquiring a second measurement value, the second measurementvalue having been obtained from inspection of a second target, anddetermining the dose parameter from the first and second measurementvalues; wherein the first and second targets comprise corresponding lineand space targets having the same pitch and inverse duty cycles.
 12. Themethod of claim 11, wherein the step of determining the dose parametercomprises: determining the dose parameter from a difference of the firstand second measurement values.
 13. The method of claim 11, wherein thestep of determining the dose parameter comprises: determining a focusparameter of the lithographic process; and referring to a dosecalibration curve corresponding to the determined focus parameter. 14.The method of claim 13, further comprising: determining the focusparameter from the first and second measurement values, wherein thefirst and second targets have been exposed with a relative best focusoffset.
 15. A lithographic system comprising: a lithographic apparatuscomprising: an illumination optical system arranged to illuminate apattern; and a projection optical system arranged to project an image ofthe pattern onto a substrate; and a metrology apparatus comprising: asupport for the substrate having a plurality of targets thereon; anoptical system for measuring each target; and a processor configured to:acquire a first measurement value, the first measurement value havingbeen obtained from inspection of a first target; acquire a secondmeasurement value, the second measurement value having been obtainedfrom inspection of a second target, wherein the first and second targetshave been exposed with a relative best focus offset; and determine thefocus parameter from a difference of the first and second measurementvalues, wherein the difference of the first and second measurementvalues is divided by an average of the first and second measurementvalues, wherein the first and second measurement values compriseintensity values of a diffraction order of radiation scattered by thefirst and second targets, respectively, wherein the lithographicapparatus is arranged to use the focus parameter and/or a dose parametercalculated by the metrology apparatus in applying the pattern to furthersubstrates.
 16. A non-transitory computer program product comprisingprocessor readable instructions which, when run on suitable processorcontrolled apparatus, cause the processor controlled apparatus toperform operations comprising: acquiring a first measurement value, thefirst measurement value having been obtained from inspection of a firsttarget; acquiring a second measurement value, the second measurementvalue having been obtained from inspection of a second target, whereinthe first and second targets have been exposed with a relative bestfocus offset; and determining the focus parameter from a difference ofthe first and second measurement values, wherein the difference of thefirst and second measurement values is divided by an average of thefirst and second measurement values, wherein the first and secondmeasurement values comprise intensity values of a diffraction order ofradiation scattered by the first and second targets, respectively. 17.The lithographic system of claim 15, wherein the diffraction ordercomprises a first diffraction order of radiation scattered from thefirst and second targets.
 18. A method of manufacturing devices whereina device pattern is applied to a series of substrates using alithographic process, the method comprising: monitoring a focusparameter by: acquiring a first measurement value, the first measurementvalue having been obtained from inspection of a first target; acquiringa second measurement value, the second measurement value having beenobtained from inspection of a second target, wherein the first andsecond targets have been exposed with a relative best focus offset; anddetermining the focus parameter from a difference of the first and thesecond measurement values, wherein the difference of the first andsecond measurement values is divided by an average of the first andsecond measurement values, wherein the first and second measurementvalues comprise intensity values of a diffraction order of radiationscattered by the first and second targets, respectively; and controllingthe lithographic process for later substrates in accordance with thedetermined focus parameter.
 19. The method of claim 18, wherein thediffraction order comprises a first diffraction order of radiationscattered from the first and second targets.